Here is the idea:
Imagine a game world where the terrain is folded in 90° angles.
Maybe six sides folded to a cube, but I can imagine even two or three sides "V" or "U" - could work, too.
Of course the twist is: reaching the corners of a terrain gravity switches and you walk "up" the walls as if nothing happened.
So in the end you get a much more vivid gampleay interaction as player contact is more dense while "physically" being far away.
This would open up a huge field of new tactics and mechanics in shooting.
The idea is way over 10 years old, but I still have screenshots of a very early development stage in a quake3 mod I started.
(Back then it was called "Power Hunters" and worked similar to ROTC but with tho goal to capture zones that are connected with energy renewal.)
(spot the tiny player in the middle):